Add Support to build loadable modules defined in mrbgems.

This is a feature request.

My aim is to build loadable modules that are mrbgem specific.
This is quite useful on cross compile. We can provide startup routines and board supports easily using this fixed mrbgem/rake system.


== My patch works like this:

=== The author of mrbgem side:

1.  writes "bins" accessor to mrbgem.rake.

```Ruby
MRuby::Gem::Specification.new('mruby-foo') do |spec|
   spec.license = 'MIT'
   spec.authors = 'mruby developers'
   spec.bins = %w(foo)
 end
```

2. puts *.c file into {gem's root}/tool/{bin's name}. In this case, {bin's name} means foo.

=== Users' side: As same as he has done.

1. Adds conf.gem into build_config.rb.
2. Executes minirake.
He will get the binary "foo" in build/{target name}/bin/.